Since he is 4, is he dry at nights yet? It is quite possible that what is causing his night terrors is the need to wee. If his parents can get to him as soon as he starts in his terrors, the sooner the better, and calmly speak to him, ask him does he need to go to the bathroom, and take his hand, walk him to the toilet and help him go potty, walk him back to bed, he will pretty much immediately fall asleep and they will have caught the terrors before they went into full blown mode. It will help train him to wake up when he needs to. They need to keep in mind that he is not awake when he is in terrors, nor will he be awake when they walk him to the bathroom. Key is to remain calm and soothing and not try to wake him out of it as you would a nightmare.
If it is a need to wee, taking him to the bathroom and putting a fresh nappy on him after he wees (he may have already done so in his nappy depending on how soon the parents respond to his cries), will help them subside and once he is back in bed he will zonk straight out.
If it is getting caught between stages of sleep, all they can really do is just be calm and soothing and making sure he doesn't hurt himself flailing about, until it subsides and he snaps out of it himself. Making sure he gets to bed at a regular time and is not over-tired or under stress will help keep episodes down. If he has a pattern, say has an episode usually an hour into sleep every time, they may try waking him gently 45 mins into sleep to interrupt his sleep cycle -- that is supposed to help. Again the sooner the parents can get to the child, whatever the root cause, the shorter the episode should tend to be.
Good luck, they are no fun but once you understand what they are they are easier to handle. And the child will grow out of it!